Effect of microstructural orientation on in situ electrical resistance monitoring during S-HDDR processing of a Nd₁₆Fe₇₆B₈ alloy

In situ electrical resistance monitoring during solid hydrogenation, disproportionation, desorption, and recombination (S-HDDR) processing of a Nd₁₆Fe₇₆B₈ alloy was carried out with the use of both longitudinal and transverse samples, with the as-cast columnar structure parallel and perpendicular to the current direction. Significant difference in the behaviour has been observed between these two kinds of samples in the first cycle of S-HDDR processing, especially during hydrogenation and disproportionation. These results have been interpreted in terms of the contribution of (1) the individual resistances of the constituent phases; (2) the particular distribution of the Nd-rich phase; (3) the factors related to hydrogen diffusion and hence to the reaction kinetics; (4) the formation of S-HDDR induced cavitation. During the second cycle of S-HDDR cycling, on the other hand, the behaviours of both samples were closely similar and this can be attributed to the formation of a non-oriented, more homogeneous, submicron microstructure and to the general re-distribution of the original Nd-rich phase after the first cycle of S-HDDR processing.